This invention relates, in general, to vapor compression systems, and more particularly, to an expansion device for a vapor compression system.
In a closed-loop vapor compression cycle, heat transfer fluid changes state from a vapor to a liquid in the condenser, giving off heat to ambient surroundings, and changes state from a liquid to a vapor in the evaporator, absorbing heat from the ambient surroundings during vaporization. A typical vapor compression system includes a compressor for pumping heat transfer fluid, such as a freon, to a condenser, where heat is given off as the heat transfer fluid condenses into a liquid. The heat transfer fluid then flows through a liquid line to an expansion device, where the heat transfer fluid undergoes a volumetric expansion. The heat transfer fluid exiting the expansion device is usually a low quality liquid vapor mixture. As used herein, the term “low quality liquid vapor mixture” refers to a low pressure heat transfer fluid in a liquid state with a small presence of flash gas that cools off the remaining heat transfer fluid as the heat transfer fluid continues on in a sub-cooled state. The expanded heat transfer fluid then flows into an evaporator. The evaporator includes a coil having an inlet and an outlet, wherein the heat transfer fluid is vaporized at a low pressure absorbing heat while it undergoes a change of state from a liquid to a vapor. The heat transfer fluid, now in the vapor state, flows through the coil outlet and exits the evaporator. The heat transfer fluid then flows through a suction line and back to the compressor. A typical vapor compression system may include more than one expansion device. Moreover, the expansion device may be placed in various locations within a vapor compression system. For example, as the heat transfer fluid flows into an evaporator it may flow through a second expansion device, where the heat transfer fluid undergoes a second volumetric expansion. Additionally, a typical vapor compression system may include a nozzle or fixed orifice.
In one aspect, the efficiency of the vapor compression cycle depends upon the precise control of the volumetric expansion of a heat transfer fluid in various locations within a vapor compression system. Heat transfer fluid is volumetrically expanded when the heat transfer fluid flows through an expansion device, such as a thermostatic expansion valve, a capillary tube, and a pressure control, or when the heat transfer fluid flows through a nozzle or fixed orifice. Often times, the rate at which a heat transfer fluid is volumetrically expanded needs to be varied depending on the conditions within the vapor compression system. Devices such as capillary tubes, pressure controls, nozzles, or fixed orifices are fixed in size and cannot vary the rate at which a heat transfer fluid is volumetrically expanded. While many thermostatic expansion valves can vary the rate at which a heat transfer fluid is volumetrically expanded, they are complex and rather costly to manufacture. Moreover, thermostatic expansion valves are not as precise as capillary tubes, pressure controls, nozzles, or fixed orifices, when it comes to controlling the rate at which heat transfer fluid is volumetrically expanded.
Accordingly, further development of vapor compression systems and of expansion devices for vapor compression systems is needed. In particular, the development of expansion devices capable of responding rapidly and precisely to variations in volumetric expansion rate are needed.